The arc-vaporization method is now widely used for the formation of fullerenes and related materials. One of the remarkable properties of fullerenes and related materials is that they have hollow structures with a cavity of nanometer-scale diameter; outer cages are made up of single or concentric multilayers of graphene sheets. When foreign materials, such as metal atom(s) or nanocrystallite, are trapped in the inner cavity, unusual physical and chemical properties are expected to be brought about. Attempts to place atoms, clusters, and nanocrystals are reviewed.